Adsorption Kinetics of As (III) from Groundwater by Nanoscale Zero-Valent Iron
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摘要: 实验室合成制得的纳米铁BET比表面积为49.16 m2/g, 直径范围为20~40 nm.通过批实验考察纳米铁对As(Ⅲ)吸附动力学情况.结果表明, 在20℃、pH为7时, 纳米铁能够快速地去除As(Ⅲ), 在60 min内, 0.1 g纳米铁对起始浓度为910 μg/L溶液As(Ⅲ)去除率大于99%.反应遵循准一级反应动力学方程, 标准化后的As(Ⅲ)速率常数kSA为2.6 mL/(m2·min).纳米铁对As(Ⅲ)的吸附等温曲线能够很好地满足Langmuir和Freundlich方程, 相关系数R2＞0.95, 由Langmuir模型获得单层纳米铁的最大吸附量为76.3 mg/g.0.1 mol/L NaOH对吸附在纳米零价铁(NZVI)的As(Ⅲ)解吸率为21%.在竞争阴离子中, SiO32-和H2PO4-对As(Ⅲ)的去除有明显阻碍作用, 而其他离子基本上没有影响.纳米铁对As(Ⅲ)的去除机理主要是吸附和共沉淀.Abstract: Artificial synthesized nanoscale zero-valent iron (NZVI) was used in the laboratory for the removal of As(Ⅲ). The average BET surface area of particles was 49.16 m2/g, with a diameter in the range of 20-40 nm. Batch experiments were carried out to study the efficiency of inorganic arsenic removal and adsorption kinetics by NZVI. The results show that As (Ⅲ) can be removed efficiently by NZVI at pH 7, 20℃. The removal rate for As (Ⅲ) is over 99% within 60 minutes by reacting 910 μg/L As (Ⅲ) with 0.1 g NZVI. The As (Ⅲ) adsorption process follows the pseudo-first-order kinetic expression. The surface-area-normalized rate coefficient kSA is 2.6 mL·m-2·min-1 for As (Ⅲ). The equilibrium adsorption data fit Langmuir and Freundlich adsorption model well, with values of the constants at the regression coefficient (R2 > 0.95) for both models. The monolayer adsorption capacity of the sorbent, as obtained from the Langmuir isotherm, 76.3 mg/g is of NZVI. 21% As (Ⅲ) adsorpted on NZVI was found to desorption by sodium hydroxide solution (0.1 M). The effect of competing anions shows SiO32- and H2PO4- markedly decrease with the removal of As (III), while the effect of other anions is insignificant. The mechanism of As removal is adsorption and coprecipitation.
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图 1 在25℃和101.3 kPa时，砷的存在形式与Eh-pH关系
Fig. 1. The Eh-pH relations for arsenic at 25℃ and 101.3 kPa
图 3 纳米铁对As(Ⅲ) Langmuir吸附等温曲线
Fig. 3. Langmuir isotherms for adsorption of As (Ⅲ) on NZVI
图 4 纳米铁对As(Ⅲ) Freundlich吸附等温曲线
Fig. 4. Freundlich isotherms for adsorption of As (Ⅲ) on NZVI
图 5 0.1 mol/L NaOH对吸附在纳米铁上的As(Ⅲ)解吸过程
Fig. 5. Desorption by of As (Ⅲ) after 21 h sorption step on NZVI by 0.1 mol/L NaOH
图 6 XPS的分析谱图NZVI(a)，吸附As(III)后的NZVI(b)
Fig. 6. XPS analysis of pristine NZVI (a), As (III) sorbed on NZVI (b)
表 1 共存离子对As(III)去除率的影响
Table 1. Effect of coexisting ions on arsenic removal by NZVI
共存离子浓度(mM) HCO3- SO42- Br- CO32- NO3- H2PO4- SiO32- 20 99.8 99.8 99.8 99.8 97.5 66.5 28.8 200 99.8 99.8 99.8 95.5 98.9 18.5 0.0 
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表 2 NZVI表面各元素含量
Table 2. Element amount of NZVI
元素 特征能谱(eV) 反应前含量(At. %) 反应后含量(At. %) C1s 284.8 29.81 17.94 O1s 530.7 55.93 56.04 Fe2p3 711.2 11.51 10.65 As3d 44.9 0 3.25 Na1s 1 071.6 0 12.11
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